1. Field of the Invention
The present invention relates to electrodes for implanting in body tissue, and more particularly to porous electrodes for pacemakers.
2. History of the Prior Art
It is known in the field of pacemakers and other medical devices in electrical communication with the tissue of the human body, to connect such devices to the body through porous electrodes which are in contact with the body tissue. Such porous electrodes promote fibrous ingrowth of the surrounding body tissue within the pores thereof, providing a semi-permanent attachment of the electrodes to the body tissue with the objective of reducing the size of the fibrotic capsule known to form around the electrode tip. As is well known, decreasing the size of the fibrotic capsule, especially in thickness, reduces the pacing threshold.
Typically, the electrode comprises a solid stud or stem, upon which a porous tip is formed. Porous tips of various configurations are old in the art. A totally "porous" design was achieved by encasing wire mesh in a basket screen (C.P.I. Models 4116, 4129-31). Other "porous" designs, such as Cordis Model "Encor," have only surface porosity. This is achieved by sintering microspheres only to the rigid metal substrate. With this technique, only limited tissue ingrowth (at electrode surface) is possible, leading to a "thick" fibrotic capsule about the electrode tip, creating a correspondingly high pacing threshold.
In the case of pacemaker electrodes, porous electrodes are designed with the objective of optimizing the somewhat conflicting requirements of small stimulation surface area and at the same time large sensing surface area. For purposes of this discussion, stimulation surface area refers to the basic external dimensions of the portion of the electrode tip implanted in the body tissue. The electrode tip is usually hemispherical in shape and the stimulation surface area refers to the area A of the hemispherical profile of the tip. By the equation J .alpha.I/A, it is recognized that current density J is inversely proportional to the stimulation surface area A. Consequently, a relatively small stimulation surface area produces a relatively high current density. For a voltage pulse of given voltage and pulse duration provided to the heart by the pacemaker, a relatively high current density enhances the likelihood of "capture" in which successful contraction of the heart takes place.
At the same time, the sensing surface area of the electrode, which is in contact with the body tissue and fluids, should be as large as possible in order to insure proper sensing. For purposes of this discussion, sensing surface area refers to the total surface area of the porous electrode tip in contact with body tissue and fluids, including the interstitial cavities of the porous tip. Sensing relates to the ability of the pacemaker to sense electrical signals generated during depolarization. Sensing sensitivity is greatly improved by the increased sensing surface area provided by a porous electrode.
Some devices of the prior art are characterized in that electrode tips are formed by adhering together a plurality of relatively thin pieces of wire. Consequently, the surface area of the tip may be increased, not however, to the extent possible with the present invention.
Although recent attempts at the use of porous electrodes has succeeded in promoting fibrous ingrowth, the level of success falls way short of that considered acceptable. In the case of some porous electrodes, the pores may be too small to provide tissue ingrowth, which would defeat the objective of minimizing fibrotic capsule formation to obtain lower pacing thresholds. Conversely, porous electrodes in which the pores are too large results in an actual lowering of the sensing area which defeats the sensing objective.
A further problem with conventional porous electrodes resides in the insufficient total surface area of the electrodes. As previously noted, pacing requires a relatively small stimulation surface area, while sensing, in turn, dictates that the total surface area of the electrode be as large as possible. While the very presence of pores in an electrode configuration will normally provide a total surface area exposed to the interfacing tissue which is many times that of the total surface area of a nonporous electrode of like size, nevertheless such total surface area is often less than it should desirably be in order to optimize pacing and sensing.
Still further problems reside in the methods currently employed to manufacture porous electrodes. Such methods are often cumbersome or inefficient or in any event fail to optimize the desired characteristics of high total surface area for an electrode of given aggregate surface area and dimension.